How to Decide on Which Fastener
Fasteners are parts which connect metal pieces and other components together. The VEX EDR system has many available options. Some of the fasteners are used when speed of assembly is needed such as in a classroom, other fasteners are used when it is important they do not loosen or break such as in a competition
All the VEX fasteners can be used in the classroom. A classroom robot may not experience as high an amount of stress as a competitive robot. The classroom robot may use fasteners which are faster to assemble like the Bearing Attachment Rivets, #8-32 Hex Nuts, Nut Bars, and Thumb Screws. These fasteners may not hold up in the interactions between robots in a competition.
Bearing Attachment Rivets are plastic rivets which consist of two parts: the Bearing Pop-rivet (Outside) and Bearing Pop-rivet (Inside). They are used as a fast attachment for bearings by inserting the pop-rivet (outside) through the bearing attachment hole and then placing the pop-rivet (inside) into the rivet to spread the rivet and hold the bearing in place.
#8-32 Hex Nuts have no locking feature. These nuts depend on pressure between the nut and the attaching surface to remain tight and a wrench needs to be used to create enough pressure.
Nut Bars are strips of unthreaded plastic nuts which are designed to allow a screw to cut threads through the nut.
Thumb Screws are screws which have a head which is designed to be tightened by hand.
|Bearing Attachment Rivets||Nut Bars|
A competition robot will need to be assembled with screws and nuts. 1-Post Nut Retainers and/or 4-Post Nut Retainers may also be used.
Screws are available in various lengths from the #8-32 x ¼” through #8-32 x 2”. There are #8-32 x ¼” and #8-32 x ½” Locking Screws, which have a nylon coating on a part of the thread which helps keep them from loosening. The screws come in two types of heads, the #8-32 Screw which is tightened with a 3/32” Hex Key and the #8-32 Star Screw which is tightened using a T15 Star Drive Key or T15 Star Screwdriver. The star type head is less prone to stripping. For pivot points there are also #8-32 Shoulder Screws which have a short unthreaded section and this area creates less friction for a pivot point.
|#8-32 Screw||#8-32 Star Screw|
|3/32” Hex Key||T15 Star Drive Key|
Nuts which are commonly used when assembling a competition robot are the #8-32 Keps Nuts and the #8-32 Nylock Nuts. The #8-32 Keps nuts have a ring of attached metal prongs which keeps the nut from loosening when the nut is tightened. The #8-32 Nylock Nuts have a nylon insert which a screw can cut threads into and this keeps the nut from loosening. The Nylock Nuts require a wrench to tighten them as the screw needs to cut its thread into the nylon. The Nylock Nuts are better for areas of vibration and are essential for creating a pivot point with a screw because the nut stays tightened by locking onto the screw’s thread instead of pressure against the metal structure like the Keps nut. The pressure will keep the pivot point from rotating easily.
|Keps Nut||Nylon Nut|
Standoffs / Couplers: Standoffs are used to separate two parts from one another while creating a rigid connection. Standoffs are ¼” hexagonal pieces of metal which have a #8-32 threaded hole cut into each end. Standoffs come in various lengths between ¼” and 6”. They can be used to secure two pieces of structural metal together with #8-32 Screws. Standoffs are tightened by using a ¼” side of the Open End Wrench. Standoffs can be connected to one another using the #8-32 Coupler. Couplers are available in ½” and 1” lengths and are available in either a Star Drive or Hex Socket version.
In order to connect two Standoffs, a #8-32 Coupler is screwed into one end of the first Standoff and the second Standoff is screwed on to the other end of the coupler. In order to tighten together two standoffs together using a #8-32 Coupler, two ¼” Open End Wrenches are required. The first wrench is used to keep the first Standoff stationary and the second wrench is used to turn the second Standoff onto the coupler, tightening the two Standoffs against one another.
Shorter Standoffs are hollow, this allows a T5 Star Key or a 5/64” Hex Key to be inserted through one Standoff and into the socket on the end of a #8-32 Coupler. The key can then hold the coupler in place while the two Standoffs are being screwed together.
A Standoff can also be attached to Shaft Collar. This can be accomplished by removing the Shaft Collar’s set screw and substituting a #8-32 Coupler into the Shaft Collar and then screwing the standoff on to the coupler. In an area where there is not enough space to use a standard #8-32 Hex, Keps, or Nylock Nuts, a ¼” long Standoff can be used as a #8-32 nut.
Hex Nut Retainers have a center hole shaped to securely fit a Hex Nut, a #8-32 screw can be tightened without the need for a wrench or pliers. The 1-Post Hex Nut Retainer has a square plastic post which provides a second point of connection in addition to the screw used for the hex nut. The 4-Post Hex Nut Retainer has 4 plastic posts and provides 5 points of connection. The 4-post Hex Nut Retainer matches the hole pattern on many of the VEX metal structural pieces, but not all of them. The Hex Nut Retainers are also ideal for classroom use because they can allow for faster assembly.
|1-Post Hex Nut Retainers||4-Post Hex Nut Retainers|
Note: The VEX Robotics Competition has a game rule about “Non-VEX screws” which allows any commercially available #4, #6, #8, M3, M3.5, or M4 screw up to 2” (50.8mm) long (nominal), and any commercially available nut, washer, and/or spacer (up to 2” / 50.8mm long) to fit these screws.
Structural metal and hardware can be purchased at https://www.vexrobotics.com/vexedr/products/structure.